1. Field of the Invention
The present invention relates to a variable integrated inductor which has an inductance value that can be switched between two or more values. In one application, the variable integrated inductor is used in a voltage controlled oscillator (VCO) which is of the type that can be used in a multi-band RF radio transceiver (e.g., wireless communication devices, such as mobile telephones, pagers, laptop computers, personal digital assistants (PDAs) and the like). In other applications, the variable integrated inductor can be used in a tuned amplifier load, an impedance matching network, a digitally controlled oscillator or any type of frequency selective LC-network.
2. Description of Related Art
Referring to FIG. 1 (PRIOR ART), there is a block diagram that illustrates the basic components of a traditional direct conversion multi-band radio transceiver 100 (e.g., wireless communication device 100). The multi-band radio transceiver 100 shown includes an antenna 102, a transmit/receive (T/R) unit 104, a receive path 106, a transmit path 108 and a base-band signal processing unit 110. The receive path 106 includes a mixer 112 that is used together with a voltage controlled oscillator (VCO) 114 to down-convert a RF frequency signal, which is received by the antenna 102, to a lower frequency that is suitable for further signal processing in the base-band signal processing unit 110. The transmit path 108 includes a mixer 116 that is used together with a VCO 118 to up-convert a base-band signal, which is received from the base-band signal processing unit 110, to a higher frequency before it is transmitted by the antenna 102. Since, the RF frequency (fRF) of the received signal and the transmitted signal can vary over a very wide range (more than a factor of 2), the multi-band transceiver 100 requires that both VCOs 114 and 118 be tunable over a wide frequency range.
This type of architecture for multi-band radio transceiver 100 has worked well in the past. However, today integrated radio transceiver solutions are being required which can cover more and more frequency bands to support more multi-band and multi-standard radio architectures. This expanded functionality has been difficult to meet because the VCOs 114 and 118 shown in FIG. 1 have a limited tuning range. A discussion is provided next to explain why the VCOs 114 and 118 have a limited tuning range.
The VCOs 114 and 118 have an oscillating frequency (f0) which is set by a LC resonator circuit 120 that contains a fixed inductor 121 and a variable capacitor 123 which are connected in parallel. The oscillating frequency (fo) is given by following equation:
                              f          o                =                  1                      2            ⁢                          π              ·                                                L                  ·                  C                                                                                        Equation        ⁢                                  ⁢                  No          .                                          ⁢          1                    Because, the value of the inductor 121 is fixed this means that the tuning range of the LC resonator circuit 120 is limited to the capacitance ratio that can be achieved by adjusting the variable capacitor 123 (i.e. varicap 123 and capacitance switch 123). The limited tuning range of the LC-network 120 is not only a problem with multi-band radio transceivers 100. It is also a problem with other types of frequency selective LC-networks that can be, for example, used in tuned amplifier loads and impedance matching networks. A number of solutions which have been used in the past to address this problem are described next with respect to FIGS. 2-5.
Referring to FIG. 2 (PRIOR ART), there is a block diagram of a dual VCO 200 which has two VCOs 202a and 202b that are both connected to a multiplexer 204. Each VCO 202a and 202b has a LC resonator circuit 206a and 206b which contains a fixed inductor 205 and a variable capacitor 207 that are connected to one another in parallel. In this case, the dual VCO 200 has a total frequency range of Vout that is made up of two sub-ranges of Vout1 and Vout2 which are outputted by VCOs 202a and 202b. Although, the dual VCO 200 is relatively easy to implement, it utilizes more than twice the silicon area than is used to make the VCO 114 (for example) shown in FIG. 1. This is not desirable.
Referring to FIG. 3 (PRIOR ART), there is a block diagram of a VCO 300 which is connected to a divider 302. The VCO 300 has a LC resonator circuit 304 which contains a fixed inductor 305 and a variable capacitor 307 that are connected to one another in parallel. The addition of the divider 302 at the output of the VCO 300 where the division ratio can be set to different integer values for different output frequency bands effectively decreases the tuning range requirements on the VCO 300. However, the addition of the divider 302 causes a significant increase the current consumption, especially if the phase noise requirements are stringent. And, the addition of the divider 302 increases the total area used on the chip. Moreover, with the addition of the divider 302 it is often difficult to generate quadrature output signals for divider ratios that are not multiples of 2. None of these characteristics are desirable.
Referring to FIG. 4 (PRIOR ART), there is a block diagram of a complex feed-back frequency generation scheme that has been used to implement a fractional division of the output signal of a VCO 400. In this scheme, the VCO 400 has a LC-type resonator circuit 402 which contains a fixed inductor 403 and a variable capacitor 405 that are connected to one another in parallel. And, the VCO's output signal is input into a mixer 404 which mixes that signal with a signal that passed through the mixer 404 and was divided by an integer N in a divider 406. The drawbacks of this scheme are that it consumes more current and takes up more space on the chip than anyone of the previous solutions shown in FIGS. 2-3.
Referring to FIG. 5 (PRIOR ART), there is a block diagram of a complex feed-forward frequency generation scheme that has also been used to implement a fractional division of the output signal of a VCO 500. In this scheme, the VCO 500 has a LC resonator circuit 502 which contains a fixed inductor 503 and a variable capacitor 505 that are connected to one another in parallel. And, the VCO's output signal is input into a mixer 504 and a divider 506. The divider 506 functions to divide the output signal by an integer N and then input the divided signal into the mixer 504. The mixer 504 then mixes both the original output signal and the divided output signal and outputs signal Vout. This scheme has the same drawbacks as the feed-back scheme shown in FIG. 4 in that it consumes more current and takes up more space on the chip than anyone of the previous solutions shown in FIGS. 2-3.
Accordingly, it can be seen that there has been and is a need for a new solution which can be used to increase the tuning range of a VCO. This new solution should not suffer from the aforementioned shortcomings and drawbacks that are associated with the traditional solutions. The variable integrated inductor of the present invention is such a solution.